In the prior art there are known various rotary tabletting machines, designed to produce the tablets from powder material, these machine comprising one turret driven to rotate around its axis.
Generally, the turret has means joined thereto for positioning and batching the material to be compressed, for compressing the powdered material and for ejecting the tablets; see e.g. U.S. Pat. Nos. 2,989,781-A1, 3,677,673-A1, 3,999,922-A1, 4,108,338-A1, 4,943,227-A1.
It is also known that the production of one tablet by a tabletting machine comprises a sequence of steps, that is, a filling step in which a suitable opening is filled with an appropriate quantity of material to be compressed, a volumetric batching of the material in the said opening, optionally a precompression step, and then the subsequent compression of the material, with consequent formation of a tablet having determined thickness. Finally, a step occurs in which the tablet is ejected from the opening.
All the above mentioned steps take place during respective angular sections of a rotation of the turret, and each working cycle can be carried out in a complete round or in a less extended arc.
The pre-compression step has the purpose of reducing the problems resulting from the fact that the tablet, in its interior, keeps embedded small quantities of air that can provoke microfractures, flackings or even explosion of the same tablet during ejection.
A tabletting machine that performs the pre-compression step is known from the European Patent 0204266-B1.
In another method, that has been proposed in a more recent time, a precompression load is kept constant for predetermined time, much longer than the usual precompression and compression times.
A tabletting machine performing a sequence which includes main and auxiliary compression steps is also known from DE 2.029.094. The said compression sequence comprises one or more main compression steps of around 2000 Kg, each of whose can be followed by an auxiliary compression step, provided by guide rails, of 300 to 1200 Kg.
Tablets obtained by applying these auxiliary compression steps are harder, and appear to have a smaller elastic return after compression.
Alternative or complementary methods, like the one that provides addition of binding materials, have been applied in order to reduce the above mentioned problems, but they have not resolved them definitely.